<dom-module id="x-sub">
  <template>
    <div id="test">test</div>
  </template>
  <script type="module">
import { PolymerElement } from '../../../polymer-element.js';
class XSub extends PolymerElement {
  static get is() { return 'x-sub' }
}
customElements.define(XSub.is, XSub);
</script>
</dom-module>